Exhaust systems for internal combustion engines are typically built from passive components through which the exhaust gas passes in all operational situations and which, as a whole, form the exhaust system. In addition to pipes, a turbocharger, a catalytic converter or a muffler may also form such a component.
In recent years, systems have been added to exhaust systems allowing an active control of sound resulting from an operation of an internal combustion engine and propagating through the exhaust system. Respective systems impart a characteristic noise emission to the exhaust noise generated by the internal combustion engine and propagating through the exhaust system that is believed to fit the image of a respective manufacturer and to be popular with customers. For this purpose, synthesized sound waves are made to interfere with sound waves (exhaust noise) propagating through an exhaust system and originating from an operation of an internal combustion engine.
This is achieved by providing a sound generator being in fluid communication with the exhaust system for transferring sound into the interior of the exhaust system. The synthesized sound interferes with the sound generated by the internal combustion engine and together both sounds are then discharged through a tailpipe of the exhaust system. Respective systems may also be used for sound muffling.
For achieving a complete destructive interference between the sound waves of the exhaust noise propagating through the exhaust system and the sound synthesized by the sound generator, the sound waves originating from the loudspeaker have to match the sound waves propagating through the exhaust system in amplitude and frequency with a relative phase shift of 180 degrees. If the sound waves generated at the loudspeaker match the sound waves of the exhaust noise propagating through the exhaust system in frequency with a phase shift of 180 degrees relative to each other, but not in amplitude, only an attenuation of the sound waves of the exhaust noise propagating through the exhaust system will be achieved.
A system for actively controlling sound propagating through the exhaust system will be described below with reference to FIGS. 1 and 2.
An exhaust system 4 including a system 7 for actively controlling sound propagating through the exhaust system 4 comprises a sound generator 3 formed by a sound proof enclosure housing a loudspeaker 2 and being coupled to the exhaust system 4 in the region of its tailpipe 1 by an acoustic duct. The tailpipe 1 comprises a discharge opening 8 for discharging exhaust gas flowing through the exhaust system 4 and airborne sound propagating through the exhaust system 4 to the exterior. An error microphone 5 is provided at the tail pipe 1. Sound inside the tail pipe 1 is measured using the error microphone 5. The error microphone 5 effects the measurement in a section downstream of a region where the acoustic duct opens into the exhaust system 4 and thus provides the fluid communication between the exhaust system 4 and the sound generator 3. The term “downstream” hereby means the direction of flow for the exhaust gas within the tailpipe 1 of the exhaust system 4. In FIG. 2, arrows illustrate the direction of flow for the exhaust gas. Further components (not shown) of the exhaust system 4, such as a catalytic converter and a muffler, may be provided in-between the region providing a fluid communication between the exhaust system 4 and the sound generator 3, and the internal combustion engine 6. Loudspeaker 2 and error microphone 5 are each coupled to a controller 9. The controller 9 is further coupled to an engine control unit 6′ of an internal combustion engine 6 by a CAN bus. The internal combustion engine 6 comprises an intake system 6″. Based on the sound measured with the error microphone 5 and the operating parameters of the internal combustion engine 6 received via the CAN bus, the controller 9 computes a control signal for loudspeaker 2 to provide the desired final sound by interference with the sound propagating through the interior of the exhaust systems's 4 tailpipe 1, and supplies the control signal to loudspeaker 2. The controller may hereto use, for example, a Filtered-x, Least Mean Squares (FxLMS) algorithm and try to emit sound using the loudspeaker for reducing the error signal obtained with the error microphone down to zero (sound cancelling) or to a preset threshold value (sound control).
A drawback the above system for actively controlling sound propagating through exhaust systems is the susceptibility of the actuators (like voice coil loudspeakers) used in the sound generators for generating sound to mechanical overload. Due to the sound pressures the actuators need to provide, the actuators are already subject to high mechanical stress when operated under normal conditions. Exhaust gas flowing through the exhaust system and hitting the actuators adds to the stress. The exhaust gas flowing through the exhaust system is usually discharged through the tailpipe's discharge opening so that the pressure acting on the actuators because of the exhaust gas flowing through the exhaust system will not be too high. In case of the discharge opening of the tailpipe being temporarily or permanently plugged (which may be the case when splashing through a puddle or passing through a snowdrift, or when a part of a passive muffler's roving fiberglass insulation comes off), the total pressure of the exhaust gas flowing through the exhaust system will act on the actuators. This may damage the actuators permanently thereby destroying them. The actuators used in a sound generator are also susceptible to thermal overload, which will not be dealt with in the present document.
For preventing mechanical or thermal damaging of an actuator of a system for actively controlling sound propagating through exhaust systems it is, for instance, known from DE 10 2011 117 495.1 to modify a control signal supplied to an actuator such that the control signal may operate the actuator without any risk of damage to the actuator. A mathematical model of the actuator is used for this purpose. This state-of-the-art approach is, however, only adapted to prevent the actuator from being overloaded solely by the control signal itself, but not to inhibit an overloading of the actuator due to excessive exhaust gas pressure inside the exhaust system.